1. Field of the invention
The present invention relates to an optical compensatory film, a method for producing the same, and a polarizing plate. Additionally, the invention relates to a TN-mode liquid crystal display device with the optical compensatory film or the polarizing plate.
2. Description of the Related Art
Various optical compensatory films with optically anisotropic layers made from liquid crystal compositions as mounted on transparent supports have been proposed conventionally as optical compensatory films for use in TN-mode liquid crystal display devices.
For example, JP-A 8-50206 (Patent reference 1) proposes an optical compensatory film comprising a transparent support and an optically anisotropic layer mounted thereon, where the optically anisotropic layer is a layer comprising a compound with a discotic structure unit and having a negative birefringence, where the disc plane of the discotic structure unit tilts toward the plane of the transparent support and where the angle between the disc plane of the discotic structure unit and the plane of the transparent support varies along the depth direction of the optically anisotropic layer.
Additionally, JP-A 2002-196146 (Patent reference 2) proposes an optical compensatory film with the optically anisotropic layer on a transparent support having an in-plane retardation (Re) and a retardation along the thickness direction (Rth) within given ranges.
Further, JP-A 2001-100031 (Patent reference 3) proposes an optical compensatory film comprising a polymer film with optically positive uniaxiality or optical biaxiality, where the direction of the polymer film at the largest refractive index is substantially parallel to the polymer plane, and additionally comprising the optically anisotropic layer on the polymer film, where the direction of the polymer film at the largest refractive index is substantially parallel or perpendicular to the average direction of lines obtained by projecting the normal of the disc planes of the discotic liquid crystal molecules in the optically anisotropic layer toward the plane of the polymer film.
WO 2004/068226 (Patent reference 4) discloses a film with the increase in Re (Re inverse dispersion) and the decrease in Rth (Rth normal dispersion) following the increase of wavelength, as an optical compensatory film for use in VA-mode liquid crystal display devicees.
Meanwhile, Japanese Patent No. 3156467 (Patent reference 5), JP-A 2008-134587 (Parent reference 6) and JP-A 2008-20780 (Patent reference 7) describe liquid crystal display devicees, each comprising a pixel electrode and a liquid crystal cell in such a structure (multi-gap structure) that the interval between the liquid crystal layer and a counter electrode confronting the pixel electrode varies depending on the color of each color filter corresponding to the pixel electrode. For example, Japanese Patent No. 3156467 describes that the viewing angle of the TN-mode liquid crystal display device can be enlarged with a combination of a TN-mode liquid crystal cell of a multi-gap structure and an optically anisotropic layer satisfying the inequality formula nx>ny>nz. JP-A 2008-134587 describes the color shift of liquid crystal display devicees, particularly of OCB-mode liquid crystal display devicees, along the oblique direction thereof is improved via a combination of a liquid crystal cell of a multi-gap structure and an optical compensatory film including an optically anisotropic layer comprising a discotic compound together with a layer with the increase of Re and Rth (Re inverse dispersion, Rth inverse dispersion) following the increase of wavelength. JP-A 2008-20780 describes that the color tints of a liquid crystal display device can be improved via a combination of an optical compensatory film with the increase in Re (Re inverse dispersion) following the increase of wavelength with a liquid crystal cell of a multi-gap structure.
When such conventional optical compensatory films of constitutions as described above are practically used in TN-mode liquid crystal display devicees, however, the contrast decrease during the change of the viewing angle is large; and additionally when half tones are displayed and watched along the oblique lateral direction, the resulting image plane may sometimes be tinted yellowish. Since displayed images on television sets or TV function-equipped PC in particular are frequently watched by plural viewers at various positions, the display monitors for use in TV sets or TV function-equipped PC are not satisfactory even though the viewing angle performance thereof is satisfactory as the conventional PC display monitor.
In TN-mode liquid crystal display devicees, generally, two polarizing plates placing a cell between them are arranged while their transmission axes are perpendicular to each other. Compared with the observation case of the liquid crystal cell along the front direction, the transmission axes of the polarizing films (polarizers) observed along the oblique lateral direction are not perpendicular to each other. Therefore, light to be essentially absorbed in the polarizing plates transmits through the polarizing plates, so that the decrease of the contrast occurs when the viewing angle tilts along the oblique direction. Because no countermeasure against such phenomenon has been done so far, the contrast then has been insufficient. Due to the wavelength dispersibility of the liquid crystal in the liquid crystal cell, additionally, the optical rotation of the liquid crystal cell varies depending on the wavelength, so that yellowish tint emerges along the oblique lateral direction when half tones are displayed. No improvement of the emergence has been done so far, alike.
The present inventors made investigations. Consequently, it was revealed that the Re values of the films described in JP-A 8-50206 (Patent reference 1) and JP-A 2002-196146 (Patent reference 2), which correspond to the second optically anisotropic layer in accordance with the invention, were so small that the gap in the transmission axes of the polarizing plates could not be compensated, therefore resulting in the insufficient contrast along the oblique direction. As the films described in JP-A 2001-100031 (Patent reference 3), furthermore, two sheets of films with different optical profiles were used as the film corresponding to the second optically anisotropic layer in accordance with the invention; still further, one of the films had Re normal dispersion and Rth normal dispersion as wavelength dispersion and the other had Re inverse dispersion and Rth inverse dispersion. Hence, it was found that the color tints of half tones were not sufficiently improved.
Alternatively, WO 2004/068226 (Patent reference 4) never discloses a specific method for preparing a film with Re inverse dispersion and Rth normal dispersion, although the reference includes a description about the film. Thus, it was found that such film could never be prepared in a practical sense.
The inventors further made investigations on the liquid crystal display devicees using the liquid crystal cells of multi-gap structures, as described in Japanese Patent No. 3156467 (Patent reference 5), JP-A 2008-134587 (Patent reference 6) and JP-A 2008-20780 (Patent reference 7). Consequently, it was found that simple use of such liquid crystal cells of the multi-gap structures could insufficiently improve the viewing angle profile or could insufficiently reduce the change of the color tints.